Electron discharge device



ELECTRON DISCHARGE DEVICE Filed Dec. 1, 1950 2 Sheets-Sheet 1 //v I/ENTOR C. 7. GODDARD ATTORNEV P 27, 1955 c. T. GODDARD 2,719,246

' ELECTRON DISCHARGE DEVICE Fild Dec. 1. 1950 2 Sheets-Sheet 2 F/G. Z

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TEMPERATURE INVENTOR C. 7.' GODDARD BYW A TTOR NE Y Uited States Patent Ofiflce 2,719,246 Patented Sept. 27, 1955 ELECTRON DISCHARGE DEVICE Charles T. Goddard, Basking Ridge, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application December 1, 1950, Serial No. 198,611

10 Claims. (Cl. 315-5) This invention relates to electron discharge devices and more particularly to reflex oscillatorsof the cavity resonator type.

In the thermal tuning of suchoscillators, thermally responsive means are provided for altering the position of one of the grids of the cavity resonator, the grid being attached to a flexible diaphragm. When it is desired to operate the oscillator at very high frequencies, the size of the resonant cavity becomes quite small. The diaphragm therefore is also limited in size. For this reason the displacement of the diaphragm must be limited so that large diaphragm movements do not occur in the tuning of the oscillator.

Further, stresses which may be present in the diaphragm can, of themselves, alter the diaphragms position by being thermally released during operation of the oscillator. In most thermally tuned oscillators, it is advantageous to so choose the operating frequency range that the thermally actuated member of the tuning device shall operate considerably above the ambient temperature within the device at all desired frequencies. This large operating temperature diiferential is desired because it enhances the time rate of change of frequency when a tuning change is initiated. However in prior thermally tuned oscillators large stresses Were introduced into the diaphragm in deflecting it over a large non-useful range in moving it to the desired operating range.

It is an object of this invention to improve the tuning characteristic for thermally tuned reflex oscillators.

It is another object of this invention to minimize the diaphragm motion necessary to tune the oscillator over the required frequency range.

It is a further object of this invention to eliminate the prestressing of the diaphragm so that stresses capable of thermal release during the operation of the oscillator are not encountered.

It is a further feature of this invention to eliminate any large non-useful diaphragm deflection outside the tuning range of the oscillator.

A still further object of this invention is to obtain a tuning rate for all desired diaphragm deflections which shall be rapid, constant and equal on the heating and cooling cycles.

A still further object of this invention is to facilitate the accurate determination during manufacture of the necessary spaces and gaps in a reflex oscillator structure.

These and other objects of this invention are accomplished in one specific illustrative embodiment in which a diaphragm, whose position of flexure determines the resonant frequency of the cavity, has connected thereto a cylindrical structure which houses the repeller electrode. A flat spring determines the spacial adjustment of the diaphragm, the spring being held in position by a spring stop and support which is capable of being easily deformed to initially adjust the position of the diaphragm and therefore the cavity frequency.

The tuner comprises a triangular pyramid structure ineluding a thin walled tubing which acts as the heat-or expansivemember, a pair of thin flat straps operating at all times near the ambient temperature of the oscillator and whose length is relatively fixed and a mounting post which is suitably insulated from the expansive member and which forms the triangular base of the pyramid. The expansive member extends slightly past the apex of the pyramid and this portion is positioned adjacent a spherical end on the cylindrical structure but is separated therefrom by an accurately defined gap. The exact dimensions of this gap may be accurately determined by an adjustment of two nuts on opposite sides of the mounting post through which the expansive member extends and by which means the length of the expansive member may be altered, thereby adjusting the gap distance.

Passage of current through the legs of the triangular pyramid structure of the tuner assembly results in an increase in temperature and consequent expansion of the thermal leg or expansive member and therefore fiexure at the hinges at each end of the thermal leg. These hinges may advantageously be formed by flattening the ends of the thin walled tubing. The gradual expansion of the thermal leg will thus first close the gap between the end of the thermal leg and the spherical portion of the cylindrical structure secured to the diaphragm and then when the end bears against the spherical portion there will be a displacement of the diaphragm against the tension of the restoring spring.

It is therefore one feature of this invention that a gap be provided between the thermal tuner structure and the diaphragm assembly. The gap permits the release of stresses in the tuning assembly which may be present due to the welding or other fabricating processes without distortion of the cavity diaphragm.

The gap also permits of close attainment to the ideal tuning characteristic and eliminates the necessity for any non-useful displacement or deflection of the diaphragm. Further, because of this feature of the invention, rapid tuning at a high temperature is attained without any large motion of the diaphragm and without prestressing the diaphragm.

It is a further feature of this invention that the tuner assembly comprise a tubular leg which is flattened at each end and which has secured thereto at least one thin strap forming a second leg of a triangle, the base being formed by the mounting post which is insulated from the expansive leg and through which the mounting for the expansion leg extends. In accordance with this feature of the invention, adjustment of the position of the flat end of the expansive leg at the apex of the triangular structure may be attained by adjustment of the connecting member extending through the mounting post.

It is a still further feature of this invention that a flat spring bearing against a portion of the cylindrical structure to which the diaphragm is connected accurately positions the diaphragm and thus determines the spacial relationship Within the cavity resonator, the spring being supported by and bearing against a stop member which may be faciley adjusted as by bending, to accurately determine the initial position of the diaphragm.

A complete understanding of the invention and of the various features thereof may be gained from consideration of the following detailed description and the accompanying drawing in which:

Fig. 1 is an elevational view mainly in section of an electron discharge device illustrative of one embodiment of this invention;

Fig. 2 is an elevational view partly in section of the tuning means employed in the device of Fig. 1;

Fig. 3 is an enlarged sectional view of the diaphragm and resonator cavity of the device of Fig. 1; and

Fig. 4 is a graph illustrating tuner characteristics for the ideal tuner, the tuning means illustrated in Fig. 2, and tuning means priorly employed.

Referring now to the drawing, the electron discharge device illustrated at Fig. 1 comprises a highly evacuated enclosing vessel including a metallic, flanged cylindrical portion having an exhaust tubulation 11 at the top thereof, a base member 12 and a skirt 13 both secured, as by welding, to the flange of the portion 10. The base member 12 has attached thereto and extending therethrough a plurality of metallic eyelets 15 in which are hermetically sealed beads 16 of a vitreous material. Lead-in conductors 18 extend through and are hermetically sealed to the beads 16, being connected to terminal pins 19 as by drops of solder 20. The terminal pins 19 are carried by an insulating disc 21, which may be of a phenolic condensation product or other insulating material and which is mounted by the skirt 13.

Mounted within the enclosing vessel are an electron gun assembly 23, more fully described in Patent 2,635,207 issued to me April 14, 1953, a cavity resonator assembly 24, and a repeller assembly 25. The electron gun assembly 23 is supported from a mounting member 27 which has a central aperture therein in which is brazed the focusing electrode 28. The gun assembly 23 comprises a hollow cylindrical cathode 29 having a concave electron emissive surface 30 and a heater element 31 located internally. A beam electrode 32 is secured to the cathode cylinder by three fingers 33, being insulated from the focusing electrode 28 by an insulating ring 34 which fits in a ridge 35 of the focusing electrode. A larger insulating ring 36 surrounds the focusing electrode 28, ring 34, and beam electrode 32. The assembly is locked by a spider spring member 39 and a ring spring member 40 separated from the spider spring member 39 by a ring insulator 41, the spring members being biased towards the mounting member 27 by the bent fingers 42 of a retaining member 43 secured, as by brazing, to the mounting member 27, as more fully described in the above-mentioned application.

The cavity resonator assembly 24, as best seen in Fig. 3, comprises a metallic cavity block member 46, a diaphragm 47 supporting a grid 48 centrally, the inner end 49 of the focusing electrode 28, which end is in the form of a truncated cone and has a grid 50 across the open mouth thereof, and an impedance transforming iris 51 designed to optimize the transport of electromagnetic energy into a wave-guide output line 52. The cavity block member 46 sets adjacent the mounting member 27,

to which it is joined as by brazing, and has an aperture 54 therein defining the resonant cavity. The wave-guide output line 52 fits into a slot in the cavity block member 46 so that the wave-guide line is bounded on two sides by the block member 46, on one side by the mounting member 27, or focusing anode 28 coplanar therewith, and on the fourth side by a piece of sheet metal 55. Referring back to Fig. 1 it can be seen that the output line is tapered, the tapered portion being bounded on three sides by sheet metal pieces 55, the wave-guide output line 52 being joined to a rectangular member 57 forming a further portion of the wave-guide line. The wave-guide member 57 is secured to a cup-shaped portion 59 of the base member 12, having an encircling flange member 60 to provide a choke 61 adjacent its end 62. This end 62 is in turn adjacent a vacuum tight glass or ceramic window 63 transparent to electromagnetic energy sealed in the bottom of the cup-shaped portion 59.

The wave-guide member 57 is supported from the base member 12 by a plate 65 having a central aperture therein through which the wave-guide member 57 extends, the plate 65 being secured, as by brazing, to boththe waveguide member 57 and the base member 12. As the mounting member 27 and the cavity block member 46 are both secured to the wave-guide member 57 at its upper end, there is thus provided a central support by which the other elements of the device are mounted from the base of the vessel.

Adjacent the window 63 at the end 62 of the waveguide section 57 is a cup-shaped coupling member 67 secured, as by brazing, to the cup-shaped portion 59 of the base member 12. The coupling member 67 is advantageously cylindrical and has a rectangular aperture 68 therethrough forming a portion of the wave-guide. A choke 69 advantageously surrounds the aperture 68 adjacent the window 63. The external portion of the coupling member has threads 70 for joining to the standard wave-guide fitting of the associated equipment. An aligning pin 71 extends below the coupling member 67 and cooperates with a corresponding aperture in the fitting to assure proper orientation of the coupling member and the associated equipment.

Referring again to Fig. 3, the cavity of the cavity resonator assembly 24 comprises the internal resonator cavity 54, which is defined and its dimensions determined by the aperture in the cavity block member 46, and a choke section 66. The choke section 66 allows a large flexible and deformable diaphragm, which has a sufliciently large flexible surface to permit of the required motion for tuning the device over the desired range, to be employed with a very minute resonator cavity, which is required for very high frequencies. The choke section 66 can be designed to present an electrical short circuit to high frequency energy at the outer wall of the inner resonator 54 so that the electronic behavior of the device is almost completely determined by the inner resonator 54, the iris coupling 51 and the output wave-guide.

Further, by providing the choke section 66 in the form of a diaphragm completely mounted on the cavity block member above the internal resonator cavity 54 high frequency power generated in the inner resonator 54 can be extracted directly therefrom by means of the coupling between the inner resonator and the output waveguide 52 afforded by the iris 51. This obviates the necessity of coupling to the choke section 66 itself rather than to the inner resonator. By coupling directly to the inner resonator 54 the efficiency of energy extraction can be higher, a high efiiciency being desirable for sustained oscillations in devices operable at such short wavelengths as, for example, six millimeters.

The repeller assembly 25 comprises a cylindrical member 74 which bears against the inner portion of the diaphragm 47 adjacent the choke section 66, as best seen in Fig. 3, and has a central flange 75 and an end flange 76 thereon. A semisphere 77 having a flange 78 is mounted on the cylindrical member 74, the flange 73 being adjacent the end flange 76 of the cylindrical member '74. A flat metallic plate 79 is positioned between the two flanges 76 and 78, the flanges and the plate being secured together as by welding or brazing. The plate 79 has a central aperture 81 therein and two cars 82 extending upward, only one of which is shown in the drawing. The ears 82 support a mica or other insulating member 83, which is preferably rectangular, to which a rod 86 is atfixed by means of an eyelet 84, the rod extending through the aperture 81 in the plate 79 and axially into the cylindrical member 74.

The repeller or reflector electrode 88 is advantageously a block affixed, as by brazing, to the end of the rod 86 and has a dished surface 89 adjacent the grid 48 and tne resonator cavity 54. A disc 90 of insulating material, as of mica, is positioned around the rod 86 and on top of the repeller electrode 88 by a cylindrical sleeve 91 secured, as by brazing, to the rod 86. The disc 90 bears against the inner wall of the cylindrical member 74 and assures an accurate axial positioning of the repeller electrode.

The initial determination of the spacing between the grids 48 and 50, or in other words of the position of the diaphragm 47 and thus of the frequency of the resonator cavity is made by a spring support assembly 94, best seen in Fig. 2. 'Referring now to that figure, the assembly comprises a flat spring member 95 having a semicircular aperture at its one end so that it may fit around the cylindrical member 74 just below the flange75 to which it may be aflixed as by welding or merely bear against. The spring member 95 advantageously has bent over side portions 96 affording it additional rigidity adjacent a stop 97 which determines the flexed position of the spring member 95. The stop 97 is formed integrally with the spring support member 98 which has a groove 99 cut in its rear surface, as best seen in Fig. 1. The spring member 95 has an aperture therein so that it may be fitted over the stop 97, slid down the support 98 and then flexed to snap into the groove 99, thus holding the spring tightly in place. The base of the support member 98 is seated, as by being brazed thereto, on an extension 100 of the cavity block member 46 past the mounting member 27 The position of the diaphragm can then be initially accurately determined by slightly bending the extension 100 of the cavity block member 46 around the point 101 which is the last point of juncture of the cavity block member 46 and the mounting member 27.

Subsequent tuning of the resonator cavity is accomplished by the tuning mechanism in accordance with this invention. As best seen in Fig. 2 a long strut 104, which may be of thin walled tubing of a material such as a molybdenum stainless steel having low creep properties at elevated temperatures, extends parallel to the wave guide 57 and the mounting member 27 for a portion of the length of the enclosing vessel. This strut is supported from the wave guide 57 by a block 105 which has an aperture in one edge through which the wave guide 57 extends and to which it is secured, two insulating washers or rings 106 and 107 positioned in an aperture in the block 105, a screw 108 and nut 109 extending through the washers 106 and 107, the bottom end 112 of the strut 104 being flattened and secured to the screw 108. The strut 104 forms one leg of a tuner triangular pyramid, the other legs of the pyramid being provided by two thin fiat straps 110 and 111 which may advantageously be of a nickel-iron alloy such as Nilvor having a low thermal conductivity and which are secured at their lower ends to the sides of the block 105 and at their upper ends to the flattened end 114 of the strut 104 adjacent the semisphere 77 but separated therefrom by a distance 113. The base of the triangular pyramid is the block 105.

In the operation of the tuner, current is passed through the circuit comprising the thermal strut 104 and the two straps 110 and 111, a lead-in conductor 18 being secured to the screw 108 and the base 12 itself being electrically connected to the straps 110, 111 through the wave guide 57 and the block 105. The straps operate at all times near ambient temperature and their length remains substantially constant. However the passage of current through the thermal strut 104 results in an increase in temperature and consequent expansion of the strut. Because the straps 110 and 111 do not also so expand, this expansion of the strut 104 causes a flexure at the hinges at each end of the strut formed by the flattened portions 112 and 113 of the tubing forming the thermal strut. These hinges are sufficiently flexible to prevent any deformation or flexure in the thermal strut 104 itself.

The gradual expansion of the thermal strut 104 thus causes the end 114 of the strut to approach the semisphere 77 thereby closing the gap 113. Further heating of the strut 104 then causes the end 114 to bear against the semisphere 77 thereby causing a displacement of the repeller assembly 25 and the diaphragm 47 against the restoring spring member 95. Displacement of the diaphragm alters the frequency of the resonator cavity and thus of the device.

The advantages resulting from a tuner constructed and employed in accordance with this invention can best be appreciated by reference to Fig. 4 which shows various thermal tuner characteristics, that is, the displacement of the diaphragm as a function of the tuner temperature, for various constructions. The desired diaphragm deflection corresponding to a desired operating tuning range is indicated on the graph by the distance 116. The temperature of the thermal tuner may be raised from the ambient temperature To within the device to the initial operating temperature To by the passage of a current through the tuner, or a portion thereof, as discussed above, or by other methods known in the art, such as by thermionic heating of the tuner. When the power supplied to the thermal tuner, by whichever mechanism and means desired, is turned off or lowered to efiect a frequency change, the temperature of the thermal element of the tuner decreases through the mechanism of heat loss through radiation to the cooler portions of the device. The time rate, of change of energy, or temperature, for this transfer is proportional to the fourth power of the temperature difference between To and To. For rapid operation of the tuner, therefore, it is advantageous that the temperature To be as high as practicable above To.

Line 117 shows an ideal tuner characteristic in which there is no deflection of the cavity diaphragm in the temperature range below To but above which temperature the rate of change of diaphragm deflection with temperature is high. After the maximum desired diaphragm deflection is reached subsequent raising of the tuner temperature in the ideal characteristic would not cause additional diaphragm deflection, as shown in the drawing.

Thermal tuners priorly employed in electron discharge devices of this type had thermal characteristics such as shown by line 118 in which any rise in temperature of the tuner mechanism even up to the ambient temperature of the device caused a deflection of the diaphragm. In obtaining the temperature To to place tuner in the range desired for rapid operation, there is a large wasteful range of diaphragm deflection corresponding to that portion of the line 118 from the origin to the point 119. This non-useful and wasted deflection, not only necessitates a larger diaphragm, but generally stresses the diaphragm beyond its elastic limit and permits uncontrolled variations in the resonant frequency of the cavity as these stresses are released by over-all thermal effects.

The tuner characteristic of a tuner mechanism in accordance with this invention is shown by the line 120 and can be considered in connection with the operation of the specific embodiment of this invention disclosed. Referring again to the other figures, the electron discharge device there illustrated can operate at its highest operating frequency without any deflection or flexure of the diaphragm. This frequency can be most advantageously determined in accordance with this invention by flexing the extension of the cavity block member 46 around the point 101, as explained above. Raising the temperature of the thermal expansible strut 104, as by passing current through it and through the straps and 111, or as by electronically bombarding the strut by independent means as known to the art, from the ambient temperature T0 of the device to the initial tuning temperature To also causes no deflection of the diaphragm, resulting only in the closing of the gap 113 between the semisphere 77 and the end 114 of the strut 104. Further heating of the strut then causes a positive and definite deflection of the diaphragm 47 and a tuning over the desired range of frequencies at a rate which may be made rapid and essentially equal on both the heating and cooling cycles of tuning. While the tuning characteristic for the embodiment disclosed departs from the ideal in that further heating of the tuner will cause additional deflection of the diaphragm beyond that desired, this is not of importance as the operator can either be advised against further heating of the tuner or the tuning 7 circuit can be arranged to render it impossible for him to do so.

In accordance with a feature of this invention the point To, that is the initial temperature at which tuning occurs, can be easily determined being dependent upon the length of the distance 113 between the semisphere 77 and the end 114 of the thermal strut 104. This distance can be varied by altering the length of the strut 104 by changing the position of the screw 108 as it extends above the block 105 which is the base of the triangular pyramid structure. Thus in accordance with this invention both of the tuning variables, that is, the initial or untuned frequency of operation and the initial temperature of tuning, can be faciley predetermined and the tuning itself substantially approximate the ideal tuning characteristic.

It is to be understood that the above-described arrangements are illustrative of the application of the principles of the invention. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. An electron discharge device comprising an enclosing vessel, a wave guide section mounted by one wall of said vessel, a cavity resonator supported by said waveguide section, said resonator including a diaphragm, means for exciting said resonator positioned to one side of said resonator and supported by said wave-guide section, a member attached to said diaphragm and extending along the axis of said resonator to the other side thereof, a base member mounted by said wave guide and having portions on either side thereof, a thermally expansive strut supported by said base member in a direction perpendicular to the axial direction of said resonator and having one end adjacent said first-mentioned member but separated therefrom, and a pair of straps each extending between one of said base portions on either side of said wave-guide section and said one end of said strut, whereby passage of current through said struts and said straps causes expansion of said strut and motion of said one end in said axial direction to close the gap between said first-mentioned member and said one end and then to bear against said first-mentioned member, thereby altering the position of said diaphragm and tuning said member.

2. An electron discharge device in accordance with claim 1 wherein said device also includes spring means positioning said diaphragm and determining the initial dimensions of said resonator, the motion of said one end bearing against said first-mentioned member being against the tension of said spring means.

3. An electron discharge device in accordance with claim 1 wherein said strut is mounted by said base member by having its other end attached to a screw extending through said base member, the length of the initial gap between said one end and said first-mentioned member being determined by the length of said screw above said base member.

4. An electron discharge device comprising a mounting member, a cavity block member secured to said mounting member and having an aperture therein, said cavity block member extending beyond the edge of said mounting member, a diaphragm across said aperture, a member attached to said diaphragm, and spring means cooperating with said last-mentioned member to determine the position of said diaphragm adjacent said aperture, said spring means including a flat spring bearing against said last-mentioned member, a stop member, and a spring support member for said flat spring, said spring support member being secured to the portion of said cavity block member extending beyond the edge of said mounting member, said portoin being bendable around the line of last juncture of said mounting member and said cavity block member to alter the position of said spring means and of said diaphragm adjacent said aperture.

5. An electron discharge device comprising a mounting member, said mounting member including a focusing electrode, a cavity block member secured to said mounting member and having an aperture therein, said focusing electrode extending into said aperture, said cavity block member extending beyond the edge of said mounting member, a diaphragm across said aperture, a cylindrical repeller electrode mounting member attached to said diaphragm, said repeller electrode mounting member having a central flange on the periphery thereof, and spring means determining the position of said diaphragm adjacent said aperture, said spring means including a flat spring member having a semicircular aperture at one end, said repeller electrode mounting member fitting into said semicircular aperture and said spring member bearing against said flange, a stop member, and a spring support member for said flat spring member, said spring support member being secured to the portion of said cavity block member extending beyond the edge of said mounting member, said portion being bendable around the line of last juncture of said mounting member and said cavity block member to alter the position of said spring means and of said diaphragm adjacent said aperture.

6. An electron discharge device comprising a mounting member, said mounting member including a focusing electrode, a cavity block member secured to said mounting member and having an aperture therein, said cavity block member extending beyond the edge of said mounting member, a diaphragm across said aperture and secured to said cavity block member, said mounting member, aperture and said diaphragm defining the dimensions of a cavity resonator, means for exciting said resonator, a first member secured to said diaphragm, spring means cooperating with said first member to determine the position of said diaphragm adjacent said aperture, said spring means including a flat spring member bearing against said first member, a stop member, and a spring support for said spring member, said spring support being secured to the portion of said cavity block member beyond the edge of said mounting member, said portion being bendable around the line of last juncture of said mounting member and said cavity block member to alter the position of said spring means and of said diaphragm adjacent said aperture, and means for flexing said diaphragm to tune said cavity resonator, said last-mentioned means including a thermally expansive member separated from said first member by a determined gap and operable to move in the direction of said first member to close said gap and bear against said first member, thereby moving said first member against the tension of said spring to flex said diaphragm.

7. An electron discharge device in accordance with claim 6 wherein said thermally expansive member is a strut positioned in said device perpendicular to the axis of said resonator, one end of said strut being separated from said first member by a determined gap and operable to move in the axial direction of said resonator to close said gap and bear against said first member to move said first member against the tension of said spring to flex said diaphragm.

8. An electron discharge device comprising an enclosing vessel, a wave guide section mounted by said vessel, a cavity resonator in energy transfer relationship with said wave guide section, said resonator including a diaphragm, means for exciting said resonator positioned to one side of said resonator, a member attached to said diaphragm and extending along the axis of said resonator to the other side thereof, a base member positioned Within said vessel, a thermally expansive strut supported by said base member in a direction perpendicular to the axial direction of said resonator and having one end adjacent said first-mentioned member but separated therefrom by a determined gap, and conducting means extending from said base member to said one end of said strap, whereby passage of current through said strut and said conducting means causes expansion of said strut and motion of said one end in said axial direction to close said determined gap and then to bear against said first-mentioned member, thereby altering the position of said diaphragm and tuning said member.

9. An electron discharge device comprising a cavity resonator having means movable to alter the physical configuration thereof, a first member attached to said means, spring biasing means operatively connected to said first member for determining the initial dimensions of said resonator, and means for tuning said resonator including a thermally expansive member separated from said first member by a determined gap and operable to move in the direction of said first member to close said gap and bear against said first member against said initial biasing means.

10. An electron discharge device comprising a cavity resonator including a diaphragm, means for exciting said resonator, spring biasing means operatively connected to said diaphragm for flexing said diaphragm to an initial physical configuration, and tuning means for flexing said diaphragm including a first member secured to said diaphragm and a thermally expansive member separated from said first member by a determined gap and operable to move in the direction of said first member to close said gap and bear against said first member in a direction opposite to said biasing means, thereby flexing said diaphragm.

References Cited in the file of this patent UNITED STATES PATENTS 2,094,602 Kassner Oct. 5, 1937 2,414,496 Varian et al. Jan. 21, 1947 2,492,993 Harrison Jan. 3, 1950 2,494,693 Ekstrand et al. Jan. 17, 1950 2,513,359 Pierce July 4, 1950 2,521,719 Hildebrand Sept. 12, 1950 2,531,214 Harris et al. Nov. 21, 1950 

